Experiences of Loneliness Across the Lifespan: A Systematic Review and Thematic Synthesis of Qualitative Studies

ABSTRACT Purpose Loneliness is a fundamentally subjective experience that is common at various life stages. Studies have qualitatively explored loneliness, but a comprehensive overview is lacking. This research therefore provides a fine-grained review of studies on loneliness experiences across the lifespan. Methods A systematic review and thematic synthesis were performed on studies that qualitatively investigated experiences of loneliness in people of any age from non-clinical populations. Sensitivity analysis assessed the impact of lower-quality studies and specific age groups on the findings. Results Twenty-nine studies of 1,321 participants aged between 7 and 103 were included. Fifteen descriptive themes and three overarching analytical themes were developed: (1) Loneliness is both psychological and contextual, (2) Loneliness centres on feelings of meaningful connection and painful disconnection, and (3) Loneliness can exist in a general, pervasive sense or can relate to specific other people or relationship types. Some features were particularly pertinent to children, younger adults, and older adults, respectively. Conclusions Loneliness involves a primarily aversive psychological experience of perceived disconnection which is linked to physical, personal, and socio-political contexts and can be pervasive or relate to specific relationships or relationship types. An awareness of context, life stage, and personal experiences is essential to understand loneliness.


Introduction
Loneliness is a common experience which is associated with various adverse physical and mental health outcomes , Victor & Yang, 2012. Despite an increasing body of research focusing on loneliness, there is relatively little work exploring its lived experience. Loneliness is generally defined as an unpleasant and distressing subjective phenomenon which arises when one's desired level of social relations differs from their actual level in number or quality (Perlman & Peplau, 1981). However, the research lacks an overarching subjective perspective, which is a significant gap in the field given that loneliness is an inherently subjective experience. Unlike age, blood pressure, or other objective phenomena, loneliness can only be definitively evaluated by asking a person whether they feel lonely. Weiss (1987) argued that although existing definitions of loneliness may be helpful, they fail to reflect the actual nature of loneliness, because they define it in terms of potential causes rather than the actual experience of being lonely. Studies based on these definitions may therefore obscure the ways that loneliness is actually experienced, potentially failing to capture aspects and idiosyncrasies of the construct. Indeed, eminent researchers in the field have recently noted that there is not yet widespread consensus on definitions of loneliness (Fried et al., 2020). A robust understanding of the phenomenon is key to achieving suitable definitions and informing approaches to studying and ameliorating loneliness.
It is clear that objective aspects of a person's social network such as social isolation are not sufficient to explain loneliness (Coyle & Dugan, 2012, Matthews et al., 2016. People can also experience meaningful social relationships which cannot be objectively evidenced, for example experiencing a relationship with God . This underscores the need to better understand the subjective experience of loneliness. The use of qualitative methods is particularly appropriate to this end given that they can capture the idiosyncrasies of lived experiences. Loneliness is present at various life stages and perhaps most commonly in younger and older adulthood (Victor & Yang, 2012). Qualter and colleagues (2015) suggest that sources of loneliness differ across the lifespan in line with changes in social priorities and influences. For example, relational proximity is prioritized more in childhood, intimacy in adolescence and young adulthood, romantic partnership in adulthood, and relational and physical losses are more relevant in older adulthood. Other factors such as how individualistic a culture is have been found to influence the relationship between age and loneliness (Barreto et al., 2021). While qualitative research has explored the experience of loneliness in adults at various life stages, a holistic investigation of these experiences across the lifespan is lacking. Rokach (1988) carried out the largest study of its type, analysing accounts of 526 adults' loneliest experiences and creating a model with major elements of self-alienation, interpersonal isolation, distressed reactions, and agony. However, most respondents were aged between 19 and 45 years old and the findings may be usefully considered alongside research qualitatively examining loneliness in wider age groups (e.g., Ojembe & Ebe Kalu, 2018). While cohort studies are useful to examine change across the lifespan, synthesizing research from different age groups can also elucidate the experience of loneliness and how it may be stable or differ over the lifespan.
Existing systematic reviews have explored the conceptualizations of loneliness employed in qualitative research with adults (Mansfield et al., 2019), experiences of loneliness in young people with depression (Achterbergh et al., 2020), and experiences and ways of dealing with loneliness in older adults published in healthcare journals (Kitzmüller et al., 2018). In contrast, the present systematic review takes a data-driven approach that focuses on non-clinical populations of all ages to synthesize participants' lived experiences of loneliness. This knowledge is imperative to deepen current understandings of loneliness and inform approaches to describing, explaining, and potentially ameliorating it. This review therefore aims to offer a holistic view of the experience of loneliness across the lifespan. There is one research question: How do people describe their experiences of loneliness?

Method
The full review protocol has been published (McKenna-Plumley et al., 2020) and registered on PROSPERO (registration number: CRD42020178105). Ethical approval was not required as primary data were not collected. The review is reported in accordance with the Enhancing Transparency in Reporting the Synthesis of Qualitative Research (ENTREQ) statement (Tong et al., 2012). The inclusion criteria were as follows: (1) Studies published in English (2) Studies with a qualitative component (reported separately if quantitative methods are also used) (3) Studies of individuals of any age who describe experiences of loneliness, not including specific clinical populations, settings, or samples closely adjacent to clinical populations (see "Amendments to protocol" for details) (4) Studies which report the subjectively perceived experience of loneliness (5) Studies in which the primary focus or one of the primary focuses is experiences of loneliness. Specifically, papers were deemed to have a sufficient focus if studying loneliness experiences was a key aspect of the work rather than simply part of the output. Studies were included if authors stated a relevant aim, objective, or research question related to investigating experiences of loneliness or if loneliness experiences were clearly explored and described (e.g., relevant questions were present in an interview guide). At the title and abstract screening stage, at least one relevant sentence or information that indicated likely relevance had to be present for inclusion. (6) Studies published in peer-reviewed journals

Amendments to protocol
Through familiarization with the available evidence, several refinements were necessary to complete a thorough and well-conceptualized review. The amendments are as follows: (1) The inclusion criterion regarding clinical populations was further specified such that clinical settings and closely clinically adjacent populations were also excluded. This was refined after noting the wide range of populations in the literature, which included specific groups adjacent to clinical populations experiencing the same types of specific circumstances as those populations. Clinical populations were defined as those with a specific clinical issue included in the ICD−11 (World Health Organization, 2021). Clinical settings included, for example, hospitals and nursing homes. Samples closely adjacent to clinical populations included, for example, carers, people experiencing bereavement, intervention participants, and individuals experiencing abuse and secondary trauma.
(2) Studies in which a proportion of participants were part of a specific clinical or clinically adjacent population or setting were included if results concerning these individuals were presented separately. (3) Grey literature and books were excluded following title and abstract screening as the volume of peer-reviewed articles was sufficient to enable a substantial but thorough qualitative synthesis. (4) The plan to use the GRADE-CERQual approach to assess confidence in findings was removed as the approach is intended primarily for findings to be used in decision-making processes (Lewin et al., 2018) and is not as applicable to the many findings of the present review which stem closely from the analysis and are not highly context specific.

Information sources and search strategy
Peer-reviewed literature was gathered between the 10 th and 12 th of March, 2021 from searches of PsycINFO, MEDLINE, Scopus, Child Development & Adolescent Studies, Sociological Abstracts, International Bibliography of the Social Sciences (IBSS), CINAHL, and the Education Resource Information Center (ERIC). Literature from database inception until the search date was considered. Additional searches for grey and difficultto-locate literature were completed using Google Scholar, opengrey.eu, ProQuest Dissertations and Theses, and websites of specific loneliness organizations, although grey literature was later removed (see "Amendments to Protocol"). In collaboration with information specialists, the search strategy was developed and search terms were translated to be appropriate for each database. The search strategy was peer reviewed by an information specialist using the Peer Review of Electronic Search Strategies (PRESS) Checklist (McGowan et al., 2016). The completed PRESS checklist is provided in Additional File 1 and the search strategy for MEDLINE is provided in Additional File 2. To find additional studies, reference lists of included studies were checked, as were the reference lists of similar systematic reviews (Kitzmüller et al., 2018, Mansfield et al., 2019, Shorey & Chan, 2021, and authors of included studies were asked to refer relevant studies.

Study selection
The database search, hand-searching, and initial screening for duplicates were completed by PMP. Title and abstract screening to remove articles which were irrelevant or inapplicable to the inclusion criteria was carried out by PMP with JG independently screening 50.92% of all titles and abstracts in Rayyan (Ouzzani et al., 2016). Full-text versions of the remaining articles were screened by PMP with JG independently reviewing 50% of the fulltexts. In cases of disagreement, the two reviewers discussed the study to reach a decision about inclusion or exclusion. The reason for the exclusion at the full-text stage was recorded.

Data extraction and quality appraisal
Information about study characteristics was extracted by PMP and fully checked by JG using a purposedesigned and piloted Microsoft Excel form. Lead authors were contacted to specify missing information. In line with guidance from Thomas and Harden (2008), all text labelled as "results" or "findings" was extracted and entered into NVivo software for analysis.
Quality of the included articles was assessed by PMP using the Joanna Briggs Institute (JBI) Critical Appraisal Checklist for Qualitative Research (Lockwood et al., 2015). Lower-quality studies were included in the review but taken into account during sensitivity analysis.

Analysis
Thematic synthesis was used to synthesize the results of the review. In thematic synthesis, descriptive themes which remain close to the primary study findings are developed. Following this, analytical theme development can be undertaken to go beyond the interpretations of primary studies and develop higher-order explanations (Thomas & Harden, 2008).
The data were coded line-by-line by PMP with codes created inductively to represent the data. Most codes represented semantic features of the data; some captured more latent aspects. Studies were coded in alphabetical order by first author's surname and then a second time working backwards from the alphabetical midpoint to check, add to, and refine codes. This ensured a relatively equal focus of coding across studies. Codes were systematically and inductively organized into descriptive themes which remained relatively close to the data. Following this, the descriptive themes were further interpreted to develop analytical themes representing overarching answers to the research question, going beyond the findings of the original studies. The process was recursive, with codes and candidate themes refined and revised as the analysis developed. Final themes were established through discussion between PMP and JG. Illustrative quotes from original study participants are provided in italics to remain grounded in the data. The relationship between codes, descriptive themes, and analytical themes is visualized in Figure 1, showing an example of the descriptive themes which primarily contributed to the development of one analytical theme and the codes underpinning one of these descriptive themes.
Following synthesis, sensitivity analyses were carried out to assess if and how (i) lower-quality studies and (ii) studies of specific age groups (children, young adults, middle-aged adults, and older adults) impacted the synthesis. This involved removing studies with specific characteristics from the database to assess whether the synthesis is altered by their removal, in terms of any themes being lost entirely or becoming less rich or thick (Carroll & Booth, 2015). We respectively removed lowerquality studies and studies of specific age groups to assess the impact of these characteristics on the findings.

Results
Of the 14,276 citations retrieved by the search, 7,065 were duplicates, leaving 7,211 for title and abstract screening. After title and abstract screening, the decision to limit the review to articles published in peer-reviewed journals was made. Following these stages, 111 articles remained for full-text screening and 29 studies were included in the final review. Where multiple reports of the same study were found, the most comprehensive was included in the review. A flowchart of the number of studies included and excluded at each stage of this process is shown in the PRISMA 2020 flow diagram (Page et al., 2021) in Figure 2.
Several studies included populations that were specified by characteristics beyond age, including migrants, first-time mothers, homeless adolescents, university students in long-distance relationships, and older people living alone. Full study characteristics are presented in Table I.

Findings
Fifteen descriptive themes which synthesize the findings of the primary studies were developed. The descriptive themes are organized into three overarching domains; specifically, in describing experiences of loneliness, people tended to discuss its (i) psychological aspects, (ii) interpersonal contextual aspects, and (iii) personal contextual aspects. The fifteen descriptive themes were then further interpreted to create three analytical themes which describe higher-order explanations in response to the research question. The descriptive and analytical themes are displayed in Figure 3. The studies contributing data to each descriptive theme are displayed in Table II.

Descriptive themes
The 15 descriptive themes representing a synthesis of the primary study findings are described below under overarching domains of psychological, interpersonalcontextual, and personal-contextual aspects of the loneliness experience. Several themes include related subthemes, represented with subheadings to express distinct but cohesive parts of the wider theme.

Psychological aspects of the experience
The following four descriptive themes represent common psychological aspects of the loneliness experience as described by people who have experienced the feeling.

Loneliness is an aversive experience
Loneliness is like a nasty disease that eats us up little by little, day by day. (Cela & Fokkema, 2017)    The review also set out to consider the reasons that study authors offered for the relative shortage of qualitative work. No specific reasons were given.

Interpersonal contextual aspects of the experience
The following five descriptive themes represent aspects of the loneliness experience related to the interpersonal context.
There was also overlap. Some individuals described missing multiple types of relationships and for some the same people played multiple social roles, such as boyfriend and best friend. However, for many, loneliness stemmed from missing specific people or lacking specific types of relationships.
Well I tried a few times but you know kids don't like me, they just like kind of ignore me. (Martin et al., 2014) While these feelings were less strictly linked to specific occurrences and could potentially represent inaccurate perceptions, they were related to interpersonal experiences that left the person feeling unwanted, inferior, and alone. [W]hen seeing that everyone else find their groups and friends and have fun . . . And seeing that they enjoy their groups, one feels extra lonely then. (Hemberg et al., 2021) Loneliness often resulted from social comparison. These comparisons were drawn by the lonely person between themselves and others, for example when noticing others who seemed more socially active or satisfied (Barke, 2017, Dahlberg, 2007, Davidson, 1995, Firmin et al., 2014, Hemberg et al., 2021, Karnick, 2008, Lee et al., 2019, McInnis & White, 2001, Rönkä et al., 2018, Tiilikainen & Seppänen, 2017. Loneliness could also arise when comparing one's current social situations to a previous more socially satisfying period (Cela & Fokkema, 2017, Firmin et al., 2014, Heravi-Karimooi et al., 2010, Kirova-Petrova, 2000, Lee et al., 2019, Morgan & Burholt, 2020, Tiilikainen & Seppänen, 2017. Unmet social norms could also provoke these feelings, for example when one was not socializing at commonly social times like weekend nights or holidays (Dahlberg, 2007, Lee et al., 2019, Rokach, 1988, Rönkä et al., 2018, Sullivan et al., 2016.

Loneliness involves social comparison
Loneliness could also emerge when people made comparisons with unrealized possibilities and when social expectations were not met (Dahlberg, 2007, Lee et al., 2019, Morgan & Burholt, 2020, Rokach, 1988, Rönkä et al., 2018, Sawir et al., 2008, Tiilikainen & Seppänen, 2017. This happened when reality did not align with prior expectations that others would be there for them or share responsibility. Loneliness existed in relation to a context of fellowship that was missing.

Personal contextual aspects of the experience
Six descriptive themes represent aspects of the loneliness experience related to personal context.
In describing loneliness, solitude (or a chosen, enjoyed aloneness) was mentioned by some (Graneheim & Lundman, 2010, Hemberg et al., 2021, Karnick, 2008, Kristensen, 1995, Martin et al., 2014, McInnis & White, 2001, Sawir et al., 2008, Sullivan et al., 2016. When alone, people could experience rest, freedom, and peace, and being alone was preferable if one needed to recuperate or did not want to socialize. However, the valence of aloneness could vary and too much time alone could be negative. Loneliness was conflated with aloneness and solitude in some cases (Dahlberg, 2007, Karnick, 2008, Martin et al., 2014, McInnis & White, 2001, Rew, 2000. Some studies referred to "voluntary" loneliness, which may otherwise be described as solitude. Some people also described aloneness when asked about loneliness. This indicates that there may be room in descriptions of loneliness for a positive interpretation linked to chosen aloneness, which was described by some studies (see theme 1.2), or that the distinctions used in research between these constructs are sometimes used more loosely in lived descriptions of loneliness.

Loneliness has a physical dimension
Relatedly, loneliness appeared to have a physical dimension, being experienced as embodied (Dahlberg, 2007, Firmin et al., 2014, Rokach, 1988, Rönkä et al., 2018, Sullivan et al., 2016, Wright-St Clair & Nayar, 2020 and accompanied by physical symptoms in some cases (Dahlberg, 2007, Hemberg et al., 2021, Kirova-Petrova, 2000, McInnis & White, 2001, Rokach, 1988, Rönkä et al., 2018. Loneliness could be painful and weigh individuals down in a metaphorical and physical sense. Physical symptoms included impaired sensory perception, stiffness, tightness in the chest, numbness, aches, changes in appetite, and feelings of being in fight or flight. "[T]hey [children] have their own financial problems, we don't want to ask them for money for a bus ticket to go downtown. Having friends means going to a bar, taking a coffee, one time you pay for them and next time they pay for you. But without money what can we do? Where can we go?" (Cela & Fokkema, 2017) Loneliness was impacted by social and cultural norms as well as the availability of resources and services. One could be lonely in a foreign culture, where ways of socializing were unfamiliar, or in their own culture due to societal changes and lack of fit (Barke, 2017, Heravi-Karimooi et al., 2010, Kwegyir Tsiboe, 2021, Ojembe & Ebe Kalu, 2018, Rönkä et al., 2018, Tiilikainen & Seppänen, 2017. Heterogendered norms could impact the types of relationships that were available and seen as valuable and affect willingness to talk about and accept loneliness (Cela & Fokkema, 2017, Heravi-Karimooi et al., 2010, Kwegyir Tsiboe, 2021, Lee et al., 2019, McInnis & White, 2001, Morgan & Burholt, 2020, Rönkä et al., 2018, Sawir et al., 2008, Smith, 2012. Religious and cultural norms around social connection were also mentioned (Barke, 2017, Cela & Fokkema, 2017, Kwegyir Tsiboe, 2021, Rokach, 1988, Rönkä et al., 2018, Sullivan et al., 2016, Wright-St Clair & Nayar, 2020.

Loneliness is affected by the socio-political landscape
Discrimination could also impact loneliness. Some studies described ageism, racism, homophobia, and discrimination based on ethnicity (Cela & Fokkema, 2017, Kwegyir Tsiboe, 2021, Sullivan et al., 2016, Wright-St Clair & Nayar, 2020. The related social rejection and abuse impacted social networks with subsequent impacts on loneliness.
[I]nadequate local public transportation system makes it difficult to get access to hospitals, doctors, relatives, religious centres and cultural centres. (Heravi-Karimooi et al., 2010) A lack of government provision and its resultant impact on the ability to connect were relevant. A lack of community and recreational activities was implicated in loneliness (Heravi-Karimooi et al., 2010, Kwegyir Tsiboe, 2021, Ojembe & Ebe Kalu, 2018, Park et al., 2019. For others, policy had direct implications, such as unaccompanied young migrants whose housing regulations and changing guardianship made it difficult to sustain relationships (Herz & Lalander, 2017, Kwegyir Tsiboe, 2021, Lee et al., 2019, McKenna-Plumley et al., 2021, Ojembe & Ebe Kalu, 2018, Park et al., 2019. In one study, older adults invoked the better financial support available in New Zealand as worthy compensation for the isolation they experienced there as migrants (Park et al., 2019). Inadequate government services were directly invoked by some (Kwegyir Tsiboe, 2021, Ojembe & Ebe Kalu, 2018. Social and political barriers to connection including discrimination, lack of fit with dominant norms, and inadequate provision of resources and services influenced loneliness.

Analytical themes
Three analytical themes were developed from the descriptive themes to address the research question ("How do people describe their experiences of loneliness?") at a holistic level.

Loneliness is both psychological and contextual
Loneliness appears to be a psychological state that is also inherently related to one's interpersonal and personal context, existing in relation to the physical, social, relational, cultural, and political context of the lonely person. While loneliness is intrinsically of the mind-an aversive emotional experience with cognitive and perceptual features-it is not isolated from the interpersonal and societal conditions of the lonely person. It is necessary to stress the separation of objective conditions of social isolation and the subjective experience of loneliness, but it appears that loneliness is still intimately connected to one's perceptions and experiences of the social world around them, as well as the conditions of their life. There was an overarching sense that personal psychology and the contextual aspects of one's life were central to descriptions of loneliness experiences. Feelings of sadness, cognitive appraisals, and social comparisons were all features of loneliness, but loneliness arose in relation to circumstances such as social transitions, physical decline, discrimination, and lack of resources which could be related to specific life stages. Loneliness is not experienced in a vacuum despite its subjective nature; although it is characterized by psychological features including a set of feelings and perceptions, it is also grounded in objective circumstances.

Loneliness centres on feelings of meaningful connection and painful disconnection
Although being alone and isolated are related to loneliness, it is subjective feelings of connection and the feeling of aloneness that are central to the experience. Loneliness involves feelings of disconnection which are aversive, unwanted, and beyond one's control, and is experienced in relation to important, meaningful relationships with other people which are felt to be lost, lacking, or absent. When people describe loneliness, they talk about important connections with other people which lessen loneliness or could if they were available. These descriptions involved discussion of faraway partners, broken friendships, busy children, and deceased spouses, as well as meaningful connections which were altogether lacking. It is one's sense that they are disconnected from important others which brings about loneliness rather than just objective, superficial aspects of the social world. Loneliness is situated against a contrasting backdrop of meaningful connection.

Loneliness can exist in a general, pervasive sense or can relate to specific other people or relationship types
Some people experience loneliness due to general dissatisfaction with their relationships or a feeling of overarching disconnection from the world. However, this synthesis underlined that in many cases, loneliness is felt in relation to specific people or types of relationships. Rather than a holistic sense of isolation, it can be due to a lack of friends or a sense that likeminded people are missing from one's life. A person who is generally satisfied socially may experience loneliness when a spouse passes away or a friendship is lost. While loneliness appears to generally share some common features-a sense of aversiveness, a feeling of disconnection-it is important to bear in mind that loneliness can occur due to a multitude of factors, from the death of a spouse to the lack of same-age friends to a feeling of disconnection from society. In this respect, loneliness appears to be a multidimensional experience which can stem from various causes and require diverse, life stage-sensitive, and individually-grounded understandings.

Sensitivity analysis
Sensitivity analysis was carried out to assess the impact of lower-quality studies and specific age groups on the synthesis. Impact was assessed by considering whether any descriptive themes became less well-evidenced (for example, less rich or thick) when these studies were removed.

Did "lower-quality" studies impact the synthesis?
Study quality may not be best reflected through quality appraisal checklists (for example, if a study is reported inadequately due to word count limitations). However, for the purpose of sensitivity analysis, a cutoff point of 4 or below out of 10 on the JBI checklist was used to delineate "lower-quality" studies, as this was the level at which fewer than half of the criteria were evidenced. Five studies scored 4 or below, although in some cases it appeared that this was due to reporting.
None of the 15 descriptive themes depended exclusively on the lower-quality studies and all were contributed to by these studies. Some aspects related to migration (for example, that being in a foreign culture was lonely, that a lack of co-ethnic meeting spaces was missed, and that migration was linked to loneliness) were less thick due to the removal of studies by Cela and Fokkema (2017) and Sawir and colleagues (2008), but these elements and the overarching themes remained. No theme was largely diminished by the removal of lower-quality studies, perhaps due to the large volume of data or because of the imperfect relationship between study quality and formal quality appraisal. Other research has similarly noted a lack of impact from removing low-quality studies (Carroll & Booth, 2015).

Are any themes specific to particular age groups?
Studies which included only participants within a specific age group were excluded one by one to assess the impact of specific age groups (children [0-15 years], young adults [16-29 years], and older adults [60+]) on the synthesis. No studies included only middle-aged adults (30-59 years), so no analysis focuses on this group.

Children
Two themes were not contributed to by the five child studies, namely "Loneliness is affected by the socio-political landscape" and "Loneliness is impacted by personality and identity", indicating that these aspects of loneliness are more specific to or more recognized by adults. Studies of children also contributed minimally to themes around social comparison, physical and mental health challenges, and life experiences and transitions (possibly because children have had less time to accumulate transitions and experiences which are relevant to loneliness). None of the descriptive themes depended exclusively on child studies, although aspects related to being left out were less prevalent when these studies were removed.

Young adults
None of the descriptive themes depended exclusively on the two studies including only young adult participants and all themes were contributed to by the young adult studies. While no theme depended exclusively on these studies, the studies contributed largely to the aspect of relocation in relation to loneliness. Additionally, data which directly described heterogendered norms by name were attributable entirely to one young adult study (Rönkä et al., 2018). However, data relating the relevance of heterogendered norms and relocation to loneliness were still present from other studies.

Older adults
None of the 15 descriptive themes depended exclusively on the 11 studies including only older adult participants and all themes were contributed to by the older adult studies. Most themes were impacted by the removal of 11 of the 29 studies, but most notably, several aspects of the theme "Loneliness is affected by the socio-political landscape" were lessened or erased. The sense that activities could manage loneliness as well as codes around the need for government involvement, cultural norms around ageing, ageism, and lack of access to services were entirely lost. Similarly, codes around inadequate government services, lack of transport, and societal change were depleted. Another large impact was in the theme "Loneliness is impacted by physical and mental health challenges", where codes around bodily decline were altogether removed and aspects of lack of independence and disability were largely impacted. Poor health remained as a major factor but 10 of the 11 older adult studies had contributed to it. Similarly, in terms of life transitions, codes around bereavement, reduced opportunities for interaction, and separation from one's partner were largely impacted.

Discussion
The current study used systematic review and thematic synthesis of 29 studies to understand how loneliness is subjectively experienced across the lifespan, enhancing our understanding of loneliness by providing a novel, fine-grained synthesis of lived experiences from 1,321 participants from 7 to 103 years old. The synthesis found that loneliness is a multifaceted experience which depends both on personal psychology and one's personal, social, political, and cultural context, that it hinges on the distinction between feelings of meaningful connection and painful disconnection, and that it can be felt in a general, pervasive sense or in relation to deficits in specific relationships. Loneliness experiences centred around psychological features, interpersonal aspects, and elements of one's personal context.

Psychological features of the loneliness experience
Loneliness appears to be intrinsically psychological despite its relationship to various contextual factors. Many studies have described loneliness as a distressing feeling , Lam et al., 2021, Perlman & Peplau, 1981 but the present synthesis expands on this to illuminate the emotional, cognitive, and perceptual components of loneliness. Loneliness feels a certain way-sad, empty, and scary -and is cyclically related to other feelings like boredom. The synthesis uncovered emotional nuances of the loneliness experience which extend beyond simple distress, supporting Bound Alberti's (2018) conceptualization of loneliness as an emotion cluster, referring to its potential to include multiple separate and potentially competing emotions. Loneliness also included cognitive components including negative appraisals of oneself and one's social world. Cognitive mechanisms are clearly influential to loneliness; research with adolescents indicates that perceived, rather than actual, social acceptance mediates the relationship between self-esteem and loneliness (Vanhalst et al., 2013). Similarly, loneliness appears to be impacted by social comparison with other people, previous circumstances, or imagined alternatives. This finding is consistent with other qualitative work on loneliness trajectories (Fardghassemi et al., 2022, Morgan & Burholt, 2022 but social comparison is relatively absent from loneliness research, despite the discrepancy between desired and actual relationships being central to the most common definition of loneliness (Perlman & Peplau, 1981). The predominant cognitive theory of loneliness describes how individuals judge their social relationships against an internal desired standard (Perlman & Peplau, 1981) but these findings suggest that external comparisons also play an important role in loneliness.
Perceptual components were another psychological aspect evidenced in this synthesis. Loneliness was described as feeling cold and impacting time perception. Experimental work has demonstrated that experiences of exclusion can lower skin temperature (Ijzerman et al., 2012), which may explain why loneliness feel cold. Social exclusion and rejection also impact light and time perception, with excluded individuals perceiving higher levels of darkness (Pfundmair et al., 2019) and overestimating time intervals (Twenge et al., 2003), although this synthesis also noted experiences of time passing quickly and stopping. However, research in this area has focused on experimentally manipulated exclusion rather than subjective ratings of loneliness. While cognitive and affective elements are often described in definitions of loneliness, this synthesis suggests that perceptual elements and social comparison warrant further attention. These findings stress the importance of understanding loneliness as an experience that is characterized centrally by psychological components, despite the relevance of objective contextual circumstances.

Contextual features of the loneliness experience
The relevance of context was clear. Loneliness was impacted by a person's physical context (e.g., location, time of day), interpersonal and personal context (e.g., relational factors, life events), and sociopolitical context (e.g., social norms and service provision). Studies using ecological momentary assessment corroborate this finding, indicating that momentary changes in physical context such as being alone are associated with loneliness (Compernolle et al., 2021, Compernolle et al., 2022. Personal context was also pertinent, as loneliness was described in reference to life transitions such as bereavement, which appeared to be particularly impactful when they increased isolation, which has also been found in recent experimental research (Evans et al., 2022). These transitions are not equally distributed across the life course (for example, educational transitions are more common in younger adulthood and retirement in later life), meaning that the contextual influences which precipitate loneliness may be unevenly distributed. This may account for the differing prevalence of loneliness at different life stages (Mund et al., 2020, Victor & Yang, 2012. Indeed, Hawkley and colleagues (2022) found that age differences in life experiences and resources appear to account for age differences in loneliness, rather than age itself. Poor health and functional decline were other personal contextual factors which precipitated loneliness, particularly for older adults, partly due to their deleterious impact on social independence. The relationship between loneliness and poor health is well established (e.g.,  and it is notable that it was a prominent finding of this study given that studies of specific clinical groups were not included. Importantly, the wider socio-political context also impacted experiences of loneliness. Inadequate provision of services and resources such as community activities, money, and transport were implicated. The lack of government provision was directly raised by some. This finding indicates that policy seeking to ameliorate loneliness should consider the availability of resources and public services, rather than purely social and personal factors. Socioeconomic factors are known predictors of loneliness (Algren et al., 2020, L. Dahlberg & McKee, 2014 and this synthesis shows that they are also subjectively identified as relevant. Wider socio-political factors such as discrimination were also implicated. Discrimination is quantitatively associated with loneliness (Priest et al., 2017;Sutin et al., 2015;Świtaj et al., 2015) and in the present synthesis it appeared that loneliness came about due both to the negative feelings stemming from discrimination and its impact on social networks. It was clear from the synthesis that loneliness could fluctuate in presence and intensity and addressing structural and social drivers may be a key way of ameliorating the experience of loneliness. Research, policy, and theory should take into account that loneliness is not divorced from the lonely person's physical, personal, and socio-political context.

Conflating and distinguishing between loneliness and related constructs
The connection between loneliness and related phenomena is also relevant. In the included studies, people who had experienced loneliness often conflated it with phenomena like aloneness, isolation, and solitude. While it is separable from these experiences, loneliness was discussed alongside objective isolation from social connections, time spent alone, and pleasant experiences of solitude. The synthesis supported common conceptualizations of loneliness as aversive, and the emotional, cognitive, and perceptual components of loneliness were generally negative. However, the synthesis evidenced the potential for positive outcomes or interpretations. The experience of loneliness could provide the potential for growth and calm. While some studies referred to loneliness itself as potentially positive, this generally appeared to constitute a voluntary state of aloneness akin to Einsamkeit (de Jong Gierveld, 1998), looking on the bright side, or a mechanism similar to post-traumatic growth (Tedeschi & Calhoun, 2004), where lonely people can in some circumstances cope and accommodate the experience. The potential for positive, or at least noncatastrophic, interpretations of loneliness has been touched upon in recent feminist approaches to the topic (Magnet & Orr, 2022, Wilkinson, 2022 and from a philosophical standpoint, with Moustakas (1961) suggesting that existential loneliness can lead to selfgrowth, but has been generally overlooked in quantitative empirical literature.
The predominant curative approach to loneliness may obscure the potential for positive outcomes and scientific exploration into them. However, it appeared that the positive interpretation of loneliness identified in the review may also be linked to the common conflation of loneliness with aloneness, isolation, and solitude. While distinction between these constructs is important to capture the subjectivity and specificity of loneliness and its generally negative expression (Hauge & Kirkevold, 2010), it may be productive for research to consider the potential for positive growth from loneliness. Moreover, it is important to note that lonely people may not differentiate as strictly between these phenomena as researchers or practitioners.
Discussions of loneliness included some overlap with related phenomena, but it appeared that the key component which distinguished loneliness was the subjective feeling of disconnection in response to a lost, imagined, or desired inverse of meaningful connection. People could experience loneliness when isolated or momentarily alone, but loneliness was not a necessary outcome of these circumstances. Instead, loneliness arises when people feel that they are disconnected, separated, misunderstood, or lacking important connections. A lack of control over the amount or kind of connections that were available was also implicated. The importance of satisfying relationships, rather than just the presence of relationships, has been referred to in the literature (Coyle & Dugan, 2012, Heinrich & Gullone, 2006, Ho et al., 2023, Perlman & Peplau, 1998. This review indicates that satisfying relationships are those which are meaningful, in that they allow intimacy and understanding with important others. Loneliness policy has tended to address relationships in a general sense, encouraging connections broadly, and although meaningful social relationships are discussed in emerging policy from the Campaign to End Loneliness (2020, 2021) and Ending Loneliness Together (2020), what constitutes a meaningful relationship should be operationalized beyond "relationship quality" given that this synthesis indicates that the perception of a lack of meaningful relationships is a central feature of loneliness. The Ending Loneliness Together policy aim to create a Meaningful Relationship Framework (2020) is encouraging to this end.

Loneliness in relation to specific relationship deficits
Loneliness is often depicted as a general feeling related to overall isolation, but the present findings emphasize that the perceived lack or inadequacy of specific relationships or relationship types, such as having no close friends, the loss of a spouse, or lack of co-ethnic social network, could also cause loneliness. This aligns with Weiss's (1973) social deficit approach to loneliness which proposes two different types of loneliness based on specific social deficits: emotional loneliness results from the lack of satisfying intimate relationships, while social loneliness occurs when one feels that they lack a sufficient social network. These relationship types broadly fulfil different needs, with intimate relationships satisfying attachment needs and social networks satisfying the need for integration (DiTommaso & Spinner, 1997, Weiss, 1973. Indeed, the current study indicates that the lack of specific types of relationships, such as those which fulfil roles like romantic partner or characteristics like shared culture or age, can provoke loneliness which is not eased by the availability of other connections. This suggests that a multidimensional typology of loneliness, like the approach proposed by Weiss (1973), should be considered alongside more prevalent theoretical approaches such as cognitive (Perlman & Peplau, 1981) and evolutionary (Cacioppo et al., 2006) theories of loneliness. This finding also has important implications for intervention development, suggesting that loneliness may not be alleviated by the creation of general social connections but that in some cases, it is relationships with specific people or types of people which produce loneliness.
Loneliness can occur due to a multitude of factors and may therefore require different understandings and pathways towards alleviation. Solutions such as befriending groups may be useful for someone who is generally socially satisfied but lonely due to a lack of friends but inappropriate for someone who is lonely due to a spousal bereavement. Schemes such as social prescribing (Reinhardt et al., 2021) may benefit further from considering the specific relationships that give rise to loneliness in some service users. Loneliness is general or pervasive for some individuals, but the relevance of specific relationships and relationship types warrants further study, given that a unidimensional approach continues to dominate loneliness research and practice.

Age-relevant features of loneliness
This review additionally aimed to shed light on how loneliness is experienced at different life stages. The sources of loneliness may differ across the lifecourse (Child & Lawton, 2019, Qualter et al., 2015 and the use of sensitivity analysis in this work allowed the assessment of how specific age groups impacted the synthesis. Aspects related to the socio-political landscape and personality and identity were not described in child studies, suggesting that these features are less or not relevant to childhood loneliness. Similarly, social comparison, physical and mental health challenges, life transitions, and life experiences were related less by children, possibly because children have had less time to accumulate these experiences. Indeed, research with children who do experience ill health (Wilson et al., 2010) and transitions such as entry into foster care (Mitchell & Kuczynski, 2010) show that they are associated with loneliness. These findings indicate that for children, immediate features of the social network may be more impactful. Meanwhile, for young adults, aspects of relocation appeared to be particularly important. Young adulthood often involves a first move out of the family home and potential subsequent moves to facilitate work or study. Relocation may also carry with it developmentally salient drivers of loneliness such as isolation, increased responsibility, and identity issues (Hopmeyer et al., 2022) which were also implicated in loneliness in this review.
Various aspects of the socio-political landscape were solely attributable to older adults. Bereavement, separation from partner, and reduced opportunities for interaction were also particularly relevant to this age group, in line with a recent review of quantitative longitudinal research (L. Dahlberg et al., 2022). It appears that for children, wider societal factors are less relevant to loneliness while for older adults they are highly relevant, as are experiences related to ageing such as declines in independence and loss of loved ones. For young adults, meanwhile, it may be age-normative transitions such as relocation which are more central. However, the inclusion of only two studies of young adults may have prevented clearer insights into this age group. Additionally, the value and salience of relationship types may vary with age; the synthesis indicated that for older adults, for example, family relationships were particularly relevant, with adult children commonly discussed.
It appears that some features of loneliness are relatively invariant across the lifespan, while others are related to contextual aspects of certain life stages. Interpersonal aspects of loneliness and its aversive, emotional, cognitive, and perceptual aspects appeared to be experienced by lonely people of various ages. However, the relevance of contextual aspects varied. Physical health challenges and bereavement appeared to impact loneliness when they were experienced, but they were more commonly described by older adults. On the other hand, the impact of socio-political context was described less by children. This aligns with recent quantitative research showing that it is the unequal distribution of experiences and resources which explains age differences in loneliness . The current review also found that loneliness could follow from an accumulation of life events, pointing to the relevance of a life-course approach for understanding loneliness. While this review offers a thorough synthesis of research including participants from childhood to old age, studies utilizing longitudinal or cohort approaches would also be valuable to this end. Additionally, most age-specific research in this area appears to focus on older adults, leaving gaps in our knowledge of loneliness in other life stages, particularly middle age. However, the present findings provide a heightened understanding of aspects of loneliness which are more pertinent to specific age groups. This may inform research aiming to understand the dynamics, correlates, and consequences of loneliness at different life stages.

Limitations and future directions
While the findings of this systematic synthesis further our understanding of the lived experience of loneliness across the lifespan, there are some limitations. Studies of specific clinical or adjacent populations were not included and aspects of loneliness which are distinctive to some specific populations may therefore be underrepresented. This was done to offer a general synthesis which can be further informed by insights from specific populations rather than subsuming and potentially obscuring aspects which may be specific to them; individuals with health difficulties and disabilities were also represented through studies not focusing on specific clinical groups. However, as the findings of this review also represent the varied circumstances, aside from clinical conditions, which impact loneliness, removing this exclusion criterion may be another appropriate approach for future work. This review also included only studies published in English in peer-reviewed journals and may overlook studies in other forums and languages. Additionally, the lack of studies specific to middle-aged adults precluded specific analysis of this age group. Future research might usefully focus on this age group, as well as understudied dynamics of loneliness including perceptual alterations, the relevance of socio-political context, age-related features of lived experiences, and loneliness that relates to specific relationships or relationship types compared to more pervasive forms.

Conclusion
Loneliness is a prevalent and harmful experience which is receiving a wealth of public, policy, and research attention. Understanding the lived experience of loneliness is central to this work given that it is a fundamentally subjective experience which is common at various stages of life. This systematic review and thematic synthesis illustrates key features of loneliness experiences in people aged from 7 to 103 years old. It appears that loneliness comprises many facets but can be described in terms of three overarching features: loneliness is (1) a psychologically mediated but contextually grounded experience, which (2) centres on feelings of painful disconnection contrasted against a backdrop of meaningful connection, and (3) can be felt in a general, more pervasive sense or can relate to a lack or loss of specific relationships and types.
Additionally, it appears that while most features are not specific to a given life stage, some are more pertinent at different stages of the life course, such as functional decline in older adulthood. In providing this detailed synthesis, this study delineates key aspects of loneliness experiences across the lifespan.